Shirley Vivian Teresa Brittain Williams, Baroness Williams of Crosby, PC (née Catlin; born 27 July 1930) is a British politician and academic. Originally a Labour Member of Parliament (MP) and Cabinet Minister, she was one of the "Gang of Four" rebels who founded the Social Democratic Party (SDP) in 1981.Between 2001 and 2004, she served as Leader of the Liberal Democrats in the House of Lords and, from 2007 to 2010, as Adviser on Nuclear Proliferation to Prime Minister Gordon Brown.